New approach to N,N-dialkoxy-N′-arylureas and N,N-dialkoxycarbamates

Article abstract of DOI:10.1016/j.mencom.2011.01.021

Methanolysis of N-chloro-N-alkoxy-N'-arylureas in the presence of silver trifluoroacetate gives the corresponding N,N-dialkoxy-N'-arylureas, whereas N-chloro-N-alkoxycarbamates react with alcohols in the presence of silver trifluoroacetate to afford N,N-dialkoxycarbamates.

Full text of DOI:10.1016/j.mencom.2011.01.021

Mendeleev
Communications
Mendeleev Commun., 2011, 21, 50–52
New approach to N,N-dialkoxy-N'-arylureas and N,N-dialkoxycarbamates†
Vasiliy G. Shtamburg,*a Alexander V. Tsygankov,b Mikhail V. Gerasimenko,a Oleg V. Shishkin,c,d
Roman I. Zubatyuk,c Alexander V. Mazepae and Remir G. Kostyanovsky*f
a Ukrainian State Chemico-Technological University, 49038 Dnepropetrovsk, Ukraine.
E-mail: stamburg@gmail.com
b State Flight Academy of Ukraine, 25005 Kirovograd, Ukraine
c STC ‘Institute for Single Crystals’, National Academy of Sciences of Ukraine, 61001 Kharkov, Ukraine
d V. N. Karazin Kharkov National University, 61099 Kharkov, Ukraine
e A. V. Bogatsky Physico-Chemical Institute, National Academy of Sciences of Ukraine, 65080 Odessa, Ukraine
f
N. N. Semenov Institute of Chemical Physics, Russian Academy of Sciences, 119991 Moscow, Russian
Federation. Fax: +7 495 137 8284; e-mail: kost@center.chph.ras.ru
DOI: 10.1016/j.mencom.2011.01.021
Methanolysis of N-chloro-N-alkoxy-N'-arylureas in the presence of silver trifluoroacetate gives the corresponding N,N-dialkoxy-
N'-arylureas, whereas N-chloro-N-alkoxycarbamates react with alcohols in the presence of silver trifluoroacetate to afford N,N-dialk-
oxycarbamates.
Nucleophilic substitution of the chlorine atom in N-chloro-
N-alkoxyureas depends on the nature of substituent at other
nitrogen atom of the urea. N-Chloro-N-alkoxy-N',N'-dimethyl-
ureas,1,2 N-chloro-N-alkoxy-N'-methylureas,3,4 N-chloro-N-ethoxy-
N'-benzylurea4 and N-chloro-N-alkoxyureas5 can be converted
into the corresponding N,N-dialkoxyureas by their alcoholysis in
the presence of base. Such N-chloro-N-alkoxyureas react similarly
with sodium carboxylates to furnish N-acyloxy-N-alkoxy deriva-
tives.5 In contrast, N-chloro-N-alkoxy-N'-arylureas 1 in the
presence of strong base6 or sodium acetate4 undergo cyclization
into 1-alkoxybenzimidazol-2-ones. This cyclization may be con-
sidered as a result of intermolecular nucleophilic substitution
at the nitrogen atom. Thus, the direct conversion of N-chloro-
N-alkoxy-N'-arylureas 1 into N,N-dialkoxy-N'-arylureas 2 looked
problematic. The latter seemed to be accessible only by the
reaction of NH-N,N-dialkoxyamines with arylisocyanates.7
In the present study we have discovered that N-chloro-N-alkoxy-
N'-arylureas 1 can be directly converted to the corresponding
N,N-dialkoxy-N'-arylureas 2 by their methanolysis in the presence
of silver trifluoroacetate (Scheme 1).‡
Replacement of CF3CO2Ag by AcONa in the methanolysis
resulted in the earlier reported4 heterocyclization, which was
exemplified on transformation of compound 1g into 1-ethoxy-
6-nitro-1,3-dihydro-2H-benzimidazol-2-one 3.§
‡
N-Chloro-N-benzyloxy-N'-(4-nitrophenyl)urea 1b was obtained by
chlorinationofN-benzyloxy-N'-(4-nitrophenyl)ureawithButOClaccording
to a described procedure.4 Yield 84%, yellowish crystals, mp 89–91°C
(decomp.). 1H NMR (300 MHz, CDCl3) d: 5.10 (s, 2H, NOCH2), 7.44–7.52
(m, 5H, Ph), 7.48 [d, 2H, C(2)H, C(6)H, 3J 9.0 Hz], 7.87 (br.s, 1H, NH),
8.18 [d, 2H, C(3)H, C(5)H, 3J 9.0 Hz]. FAB MS, m/z (%): 324 [M + H]+
(11), 322 [M + H]+ (28), 91 Bn+ (100). Found (%): Cl, 10.95. Calc. for
C14H12N3O4Cl (%): Cl, 11.02.
Compounds 1c,d,f were synthesized in a similar manner by chlorination
of corresponding N-alkoxy-N'-arylureas. For their characteristics, see
Online Supplementary Materials. N-Chloro-N-alkoxy-N'-arylureas 1a,6
1e4 and 1g4 were reported earlier.
Synthesis of N,N-dimethoxy-N'-(4-nitrophenyl)urea 2a (general proce-
dure). The solution of 1a6 (0.099 g, 0.403 mmol) in CH2Cl2 (2 ml) was
added to a cooled to –27°C solution of CF3CO2Ag (0.107 g, 0.484 mmol)
in abs. MeOH (5 ml). AgCl was precipitated. The reaction mixture was
warmed to 8°C for 16 h, then AcONa (0.082 g, 1.00 mmol) was added.
After that, MeOH was evaporated in vacuo, the residue was extracted
with CH2Cl2 (15 ml). The CH2Cl2 extract was evaporated in vacuo, the
residue was extracted with benzene (15 ml). The benzene extract was
evaporated in vacuo to yield 0.091 g (93%) of product 2a, pale yellow
crystals, mp 81–83°C (benzene–hexane). 1H NMR (300 MHz, CDCl3) d:
3.97 [s, 6H, N(OMe)2], 7.72 [d, 2H, C(2)H, C(6)H, 3J 9.3 Hz], 8.18 (br.s,
Cl
N
OMe
N
H
H
MeOH, CF3CO2Ag
N
N
Ar
Ar
OR
OR
O
O
1a–f
2a–f
NH
AcONa / MeOH
1g
O
N
O2N
3
OEt
1H, NH), 8.26 [d, 2H, C(3)H, C(5)H, J 9.3 Hz]. 13C NMR (400 MHz,
CDCl3) d: 62.23 (OMe), 118.75 [C(2), C(6)], 124.84 [C(3), C(5)], 142.25
[C(1)], 143.86 [C(4)], 156.24 [NHC(O)]. FAB MS, m/z (%): 242 [M + H]+
(82), 210 [M + H – MeOH]+ (100). Found (%): C, 44.79; H, 4.83; N, 17.25.
Calc. for C9H11N3O5 (%): C, 44.82; H, 4.60; N, 17.42.
3 61%
a Ar = 4-O2NC6H4, R = Me
b Ar = 4-O2NC6H4, R = Bn
c Ar = 4-O2NC6H4, R = Me2CH(CH2)2
d Ar = 2-O2NC6H4, R = Et
Compounds 2b–f were synthesized analogously. For their charac-
teristics, see Online Supplementary Materials.
e Ar = 4-ClC6H4, R = Et
§
N-Chloro-N-ethoxy-N'-(4-nitrophenyl)urea 1g4 (0.058 g, 0.223 mmol)
f
Ar = 4-BrC6H4, R = Et
was dissolved in solution of AcONa (0.087 g, 0.333 mmol) in MeOH
(6 ml) at –30°C. The solution was heated to 15°C for 3 h and kept at
15–17°C for 24 h. Then MeOH was evaporated in vacuo, the residue was
extracted with CH2Cl2 (10 ml), CH2Cl2 extract was evaporated in vacuo,
the residue was crystallized from Me2CO–CH2Cl2, yielding 0.0304 g (61%)
of 1-ethoxy-6-nitro-1,3-dihydro-2H-benzimidazol-2-one 3, white crystals,
mp 209–211°C, identified by 1H NMR.4
g Ar = 4-O2NC6H4, R = Et
Scheme 1
†
Geminal Systems, Part 59. For the previous part, see V. G. Shtamburg,
E. A. Klots, M. V. Gerasimenko, O. V. Shishkin, R. I. Zubatuk and R. G.
Kostyanovsky, New J. Chem., in press.
© 2011 Mendeleev Communications. All rights reserved.
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Mendeleev Commun., 2011, 21, 50–52
Cl
In contrast to N-chloro-N-alkoxyureas, N-chloro-N-alkoxy-
Ag+
R' OH
X
N
X
N
OR
carbamates cannot be converted into N,N-dialkoxycarbamates
by alcoholysis in the presence of bases, which led to reduction
products such as NH-N-alkoxycarbamates and N,N'-bis(alk-
oxycarbonyl)-N,N'-dialkoxyhydrazines.8,9 However, reaction of
N-chloro-N-alkoxycarbamates with sodium acetate in MeCN
gave N-acetoxy-N-alkoxycarbamates, which can be easily trans-
formed into N,N-dialkoxycarbamates on alcoholysis.5
– AgCl
OR
A
1 X = C(O)NHAr
4 X = C(O)OMe
H
O
R'
OR'
Here we have found that N-chloro-N-alkoxycarbamates 4a–c¶
can be directly converted into the corresponding N,N-dialkoxy-
carbamates 5a–d†† by their alcoholysis in the presence of silver
trifluoroacetate (Scheme 2).
In this manner, sterically hindered O-methyl-N,N-diisopropyl-
oxycarbamate 5c is obtained with moderate yield by isoprop-
X
N
X
N
– H+
OR
OR
2 X = C(O)NHAr
5 X = C(O)OMe
Scheme 3
Cl
OR'
anolysis of compound 4a. Note that isopropanolysis of O-ethyl-
N-acetoxy-N-metoxycarbamate5 does not lead to N,N-dialkoxy-
carbamate.
R' OH, CF3CO2Ag
MeO(O)C
N
MeO(O)C
N
OR
OR
Earlier Glover10–12 and Kikugawa13,14 proposed the general
method for the synthesis of O,N-containing heterocycles by
intramolecular cyclization of N-chloro-N-alkoxycarboxamides
in the presence of silver salts, which proceeded via generation
of N-alkoxynitreniun ions.10–14 Probably, in our case formation
of N,N-dialkoxy-N'-arylureas 2 and N,N-dialkoxycarbamates 5
also proceeds via the step of N-alkoxynitreniun ions A (Scheme 3).
Previously,15 the structure of the simplest N,N-dimethoxy-
urea has been studied. Herein, the XRD study of N,N-dimethoxy-
N'-(4-nitrophenyl)urea 2a was performed (Figure 1).‡‡ The degree
of pyramidalization at amide nitrogen atom N(1) in urea 2a is
high enough, as the sum of bond angles centered on this nitrogen
atom is 324.0(2)°, the deviation of N(1) atom from the plane of
bonded atoms is 0.508(3) Å. This degree of pyramidalization is
close to those of N-acyloxy-N-alkoxybenzamides16 and N-acyloxy-
N-alkoxyureas.15
4a R = Pri
4b R = Et
4c R = C8H17
5a R = Pri, R' = Me, 58%
5b R = Pri, R' = Et,
59% (from 4a),
45% (from 4b)
5c R = R' = Pri, 59%
5d R = C8H17, R' = Me, 76%
Scheme 2
¶
Methyl N-chloro-N-isopropoxycarbamate 4a has been synthesized by
chlorination of methyl N-isopropoxycarbamate with ButOCl by reported
procedure,5 yellowish liquid. 1H NMR (300 MHz, CDCl3) d: 1.28 (d, 6H,
NOCHMe2, 3J 6.3 Hz), 3.91 (s, 3H, CO2Me), 4.31 (sept., 1H, NOCHMe2,
3J 6.3 Hz). IR (n/cm–1): 1780 (C=O). Found (%): Cl, 21.04. Calc. for
C5H10ClNO3 (%): Cl, 21.15.
Methyl N-chloro-N-ethoxycarbamate 4b has been synthesized by chlor-
ination of methyl N-ethoxycarbamate with ButOCl by reported procedure,5
yellowish liquid. 1H NMR (300 MHz, CDCl3) d: 1.31 (t, 3H, NOCH2Me,
3J 6.9 Hz), 3.92 (s, 3H, CO2Me), 4.07 (q, 2H, NOCH2Me, J 6.9 Hz).
3
The N(1)–C(1) amide bond is much longer [1.441(3) Å] com-
pared to the N(2)–C(1) amide bond [1.357(3) Å] owing to the
higher conjugation of the planar N(2) atom with carbonyl group.
IR (n/cm–1): 1795 (C=O). Found (%): Cl, 22.85. Calc. for C4H8ClNO3 (%):
Cl, 23.09.
Methyl N-chloro-N-octyloxycarbamate 4c was reported earlier.5
†† Synthesis of methyl N,N-diisopropoxycarbamate 5c (general procedure).
Methyl N-chloro-N-isopropoxycarbamate 4a (0.673 g, 4.017 mmol) was
dissolved in PriOH (2 ml) at cooling to –27°C, and the obtained solution
was rapidly added to the solution of CF3CO2Ag (1.065 g, 4.821 mmol)
in PriOH (5 ml) at –27°C, the reaction mixture was warmed to 11°C for
19 h, then AcONa (0.46 g, 5.61 mmol) was added, the mixture was stirred
for 2 h, and the solid formed was filtered off. The filtrate was concentrated
in vacuo, the residue was twice extracted with the mixture of CH2Cl2 (7 ml)
and C6H14 (5 ml). The combined extracts were evaporated in vacuo. The
residue was distilled in vacuo to afford 0.456 g (59%) of product 5c,
colourless liquid, nD20 1.4189. 1H NMR (300 MHz, CDCl3) d: 1.29 (d, 12H,
NOCHMe2, 3J 6.3 Hz), 3.85 (s, 3H, CO2Me), 4.28 (sept., 2H, NOCHMe2,
3J 6.3 Hz). Found (%): C, 50.31; H, 8.71; N, 7.08. Calc. for C8H17NO4 (%):
C, 50.25; H, 8.96; N, 7.32.
O(4)
O(3)
C(4)
C(8)
N(1)
C(5)
C(6)
C(3)
C(2)
O(1)
N(2)
N(3)
C(1)
O(5)
C(7)
C(9)
O(2)
Figure 1 Crystal structure of N,N-dimethoxy-N’-(4-nitrophenyl)urea 2a.
Selected bond lengths (Å) and bond angles (°): O(1)–N(1) 1.418(3), O(2)–N(1)
1.412(3), O(1)–C(8) 1.428(3), O(2)–C(9) 1.437(3), O(3)–C(1) 1.204(3),
N(1)–C(1) 1.441(3), N(2)–C(1) 1.357(3), N(2)–C(2) 1.401(3); O(2)–N(1)–
O(1) 106.46(19), O(1)–N(1)–C(1) 108.0(2), O(2)–N(1)–C(1) 109.5(2), O(3)–
C(1)–N(2) 126.8(2), O(3)–C(1)–N(1) 119.6(2), N(2)–C(1)–N(1) 113.4(2).
‡‡ Crystal data for 2a. Crystals were grown from CH2Cl2–C6H14 at –20°C,
C9H11N3O5, M = 241.21, monoclinic, space group P21/c, at 100 K,
a = 4.8371(3), b = 16.9591(11) and c = 12.7986(9) Å, b = 92.609(7)°,
V = 1048.82(12) Å3, F(000) = 504, dcalc = 1.528 g cm–3, Z = 4, m =
= 0.126 mm–1. Data were measured using an Xcalibur 3 diffractometer
(graphite-monochromated MoKa radiation, 2q/q scan). Selected crystal
is found to be a non-merohedral twin due to 180° degree rotation along the
a axis with relative contributions of twin components of 0.58:0.42. Total
9049 reflections were measured up to 2qmax = 57.74°, of which 4131 are
unique (Rint = 0.072). The structure was solved by direct method using
the SHELX-97 program package.20 Refinement against F2 in an anisotropic
approximation (the hydrogen atoms isotropic in the riding model) by a
full matrix least-squares method for 4037 reflections was carried out to
wR2 = 0.145 [R1 = 0.059 for 2316 reflections with F > 4s(F), S = 0.98].
CCDC 776941 contains the supplementary crystallographic data for
this paper. These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
For details, see ‘Notice to Authors’, Mendeleev Commun., Issue 1, 2011.
Methyl N-isopropoxy-N-methoxycarbamate 5a was obtained similarly
to compound 5c by methanolysis of 4a, yield 58%, colourless liquid,
bp 50–53°C (3 Torr.), nD23 1.4168. 1H NMR (300 MHz, CDCl3) d: 1.30 (d,
6H, NOCHMe2, 3J 6.0 Hz), 3.79 (s, 3H, NOMe), 3.87 (s, 3H, CO2Me),
4.28 (sept., 1H, NOCHMe2, 3J 6.0 Hz). IR (n/cm–1): 1770 (C=O). EI MS,
m/z (%): 163 M+ (3.4), 105 (5.6), 91 (14.0), 60 (21.3), 59 (54.8), 58 (24.3),
46 (16.9), 45 (36.7), 44 (21.3), 43 (100). Found (%): C, 44.23; H, 8.17;
N, 8.42. Calc. for C6H13NO4 (%): C, 44.17; H, 8.03; N, 8.58.
Methyl N-ethoxy-N-isopropoxycarbamate 5b was obtained similarly to
compound 5c by ethanolysis of 4a, yield 59%, and by isopropanolysis of
4b, yield 45%, colourless liquid, nD21 1.4200. 1H NMR (300 MHz, CDCl3)
d: 1.293 (t, 3H, NOCH2Me, 3J 7.2 Hz), 1.295 (d, 6H, NOCHMe2, 3J 6.3 Hz),
3
3.86 (s, 3H, CO2Me), 4.06 (q, 2H, NOCH2Me, J 7.2 Hz), 4.28 (sept.,
1H, NOCHMe2, 3J 6.3 Hz). Found (%): C, 47.19; H, 8.67; N, 7.74. Calc.
for C7H15NO4 (%): C, 47.45; H, 8.53; N, 7.90.
Methyl N-methoxy-N-octyloxycarbamate 5d5 was obtained similarly to
compound 5c by methanolysis of 4c,5 yield 75%, and was identified by
1H NMR.
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Mendeleev Commun., 2011, 21, 50–52
2 V. F. Rudchenko, V. I. Shevchenko and R. G. Kostyanovskii, Izv. Akad.
Such difference between the N–C amide bonds is also observed
for unsubstituted N,N-dimethoxyurea,15 N-chloro-N-alkoxyureas4
and N-acyloxy-N-alkoxyureas.15,17 The lengths of the N(1)–O(1)
and N(1)–O(2) bonds [1.418(3) and 1.412(3) Å, respectively] are
somewhat greater than those of the N–O bonds in N,N-di-
methoxyurea (1.397 and 1.401 Å15).
The lone pair (Lp) of the N(1) atom has orthogonal orientation
with respect to the amide group plane [the O(3)–C(1)–N(1)–
Lp(N1) torsion angle is 83°]. Contrary to N,N-dimethoxyurea,15
the both methoxy groups are oriented towards the LpN(1): the
C(8)–O(1)–N(1)–Lp(N1) and the C(9)–O(2)–N(1)–Lp(N1) torsion
angles are 7° and 33°, respectively. The phenyl group is coplanar
to the amide one [the C(3)–C(2)–N(2)–C(1) torsion angle is
5.4(4)°]. The nitro group is slightly turned round the cycle plane
[the C(4)–C(5)–N(3)–O(4) torsion angle is 13.4(4)°].
Nauk SSSR, Ser. Khim., 1986, 598 (Bull. Acad. Sci. USSR, Div. Chem.
Sci., 1986, 35, 543).
3 V. F. Rudchenko, S. M. Ignatov and R. G. Kostyanovskii, Izv. Akad. Nauk,
Ser. Khim., 1992, 2441 (Bull. Russ. Acad. Sci., Div. Chem. Sci., 1992,
41, 1920).
4 V. G. Shtamburg, O. V. Shishkin, R. I. Zubatyuk, S. V. Kravchenko,
A. V. Tsygankov, A. V. Mazepa, E. A. Klots and R. G. Kostyanovsky,
Mendeleev Commun., 2006, 323.
5 V. G. Shtamburg, E. A. Klots, A. P. Pleshkova, V. I. Avramenko, S. P.
Ivonin, A. V. Tsygankov and R. G. Kostyanovskii, Izv. Akad. Nauk, Ser.
Khim., 2003, 2132 (Russ. Chem. Bull., Int. Ed., 2003, 52, 2251).
6 J. Perronet and J-P. Demoute, Gazz. Chim. Ital., 1982, 112, 507.
7 V. F. Rudchenko, S. M. Ignatov and R. G. Kostyanovskii, Izv. Akad. Nauk
SSSR, Ser. Khim., 1989, 2384 (Bull. Acad. Sci. USSR, Div. Chem. Sci.,
1989, 38, 2195).
8 V. G. Shtamburg, V. F. Rudchenko, Sh. S. Nasibov, I. I. Chervin and
R. G. Kostyanovskii, Izv. Akad. Nauk SSSR, Ser. Khim., 1981, 449 (Bull.
Acad. Sci. USSR, Div. Chem. Sci., 1981, 30, 423).
9 V. G. Shtamburg, V. M. Grinev, E. A. Klotz and A. V. Tsygankov, Visnik
Dnipropetrovskogo Universitetu, Khimiya, 2005, 11, no. 7, 104 (in
Ukrainian).
The observed molecule conformation is probably additionally
stabilized by weak intramolecular hydrogen bonds N(2)–H(2)···O(2)
(H···O 2.13 Å, N–H···O 108°) and C(3)–H(3)···O(3) (H···O 2.24 Å,
C–H···O 122°).
10 S. A. Glover, A. Goosen, C. W. McCleland and J. L. Schoonraad, J. Chem.
Soc., Perkin Trans. 1, 1984, 2255.
11 S. A. Glover, A. Goosen, C. W. McCleland and J. L. Schoonraad,
Tetrahedron, 1987, 43, 2577.
12 S. A. Glover, C. A. Rowbottom and A. P. Scott, Tetrahedron, 1990, 46,
7247.
13 Y. Kikugawa and M. Kawase, J. Am. Chem. Soc., 1984, 106, 5728.
14 M. Kawase, T. Kitamura andY. Kikugawa, J. Org. Chem., 1989, 54, 3394.
15 V. G. Shtamburg, O. V. Shishkin, R. I. Zubatyuk, S. V. Kravchenko, A. V.
Tsygankov, V. V. Shtamburg, V. V. Distanov and R. G. Kostyanovsky,
Mendeleev Commun., 2007, 17, 178.
In the crystal molecules of N,N-dimethoxy-N'-(4-nitrophenyl)-
urea 2a are linked by intermolecular hydrogen bonds N(2)–
H(2)···O(4') [–1 + x, 0.5 – y, –0.5 + z] (H···O 2.27 Å, N–H···O
155°) and stacking interactions between p-systems of two mole-
cules connected by a translation along the a crystal axis [the phenyl
ring center lies by 3.50 Å above the center of the C(1)–N(1) bond].
Thus, the nitrenium ions generation from N-chloro-N-alkoxy-
N'-arylureas and N-chloro-N-alkoxycarbamates by the action of
silver trifluoroacetate in alcohol media allows for the straight-
forward access to N,N-dialkoxy-N'-arylureas and N,N-dialkoxy-
carbamates, respectively. These kinds of N,N-dialkoxyamides are
known as the starting compounds in the synthesis of valuable
NH-N,N-dialkoxyamines.18,19
16 A.-M. E. Gillson, S. A. Glover, D. J. Tucker and P. Turner, Org. Biomol.
Chem., 2003, 1, 3430.
17 O. V. Shishkin, R. I. Zubatyuk, V. G. Shtamburg, A. V. Tsygankov, E. A.
Klots, A. V. Mazepa and R. G. Kostyanovsky, Mendeleev Commun.,
2006, 222.
18 V. F. Rudchenko, S. M. Ignatov, I. I. Chervin, V. S. Nosova and R. G.
Kostyanovskii, Izv. Akad. Nauk SSSR, Ser. Khim., 1986, 1153 (Bull.
Acad. Sci. USSR, Div. Chem. Sci., 1986, 35, 1045).
19 V. G. Shtamburg, A. V. Tsygankov and A. P. Pleshkova, Visnik Dnipro-
petrovskogo Universitetu, Khimiya, 2007, 13, no.10/2, 75 (in Ukrainian).
20 G. M. Sheldrick, Acta Crystallogr., 2008, A64, 112.
This work was supported by the RussianAcademy of Sciences
and the Russian Foundation for Basic Research (grant no. 09-03-
00537a).
Online Supplementary Materials
Supplementary data associated with this article can be found
in the online version at doi:10.1016/j.mencom.2011.01.021.
References
1 V. F. Rudchenko, V. I. Shevchenko, S. M. Ignatov and R. G. Kostyanovskii,
Izv. Akad. Nauk SSSR, Ser. Khim., 1983, 2411 (Bull. Acad. Sci. USSR, Div.
Chem. Sci., 1983, 32, 2174).
Received: 2nd June 2010; Com. 10/3536
– 52 –